1. Field of the Invention
The present invention relates to a technique of making oblique stripes in a beat noise form generated when interlaced driving of a liquid crystal inconspicuous by devising the arrangement of a micro color filter in a color liquid crystal display unit.
2. Description of the Related Art
In the National Television System Committee (NTSC) system known as one of standard systems of television, the number of scanning lines of 525 and 30 frames per second are adopted. In general, however, in order to reduce blinking of a picture called flicker, interlaced scanning is employed in which one frame is divided into two parts, an even-numbered field and an odd-numbered field, and respective fields are scanned in a consecutive order so as to scan them alternately.
When the interlaced scanning is performed, an even-numbered scanning line is displayed in an even-numbered field and an odd-numbered scanning line is displayed in an odd-numbered field. By doing so, it is possible to double an apparent scanning cycle, and the flicker becomes inconspicuous. Further, since the actual effective frame is approximately 480 lines in the number of scanning lines and is approximately 80% in the horizontal direction, the pixel structure is manufactured as one having 480.times.640 in full resolution products, what is called a 480-line structure, and as one having approximately 240.times.400 in half resolution products (only half of the number of scanning lines is used), what is called a 240-line structure in a general liquid crystal display unit. Further, a system that adopts the interlaced scanning in a liquid crystal display unit is to be called an interlaced driving system here.
Here, the scan driving indicates that a pixel line in a certain direction is selected and a display signal is applied to that line. In a liquid crystal display unit, however, when direct-current driving is performed when a signal is applied, ions in the liquid crystal are accumulated in a substrate on one side and liable to cause deterioration of the display unit.
Therefore, in order to prevent such a problem, the display signal applied to the liquid crystal is inverted in polarity positive to/from negative for every field in driving.
Further, in the interlaced scanning in a television of the NTSC system, one frame (one scope) is constituted by putting the odd field (the first field) and the even field (the second field) together, but a field frequency is set to 60 Hz and a frame frequency is set to 30 Hz in general.
The scan driving state is shown in FIG. 6. In FIG. 6, numerical numbers (23 to 263) on the left side show scanning line numbers in the first field, and numerical numbers (285 to 525) on the right side show scanning line numbers in the second field. Besides, the fact that the scanning line is shown to decline from left to right means that scanning is performed from the upper left part to the lower right part following scanning lines running from the top to the bottom of the frame, thus forming a picture of one frame or one field.
As against the above, in a liquid crystal display unit in the present state a typical television picture display state of 240 lines is driven by a single line sequential driving system in which, as shown in FIG. 7, a write frequency is set to 60 Hz, scanning signals of the first field are set to the numbers 23, 24, 25 to 260, 261 and 262, and scanning signals of the second field are set to the numbers 286, 287, 288 to 523, 524 and 525, thus writing signals in the first field and the second field putting them on the same scanning line.
Further, as shown in FIG. 8, as a display state of a typical television picture of 480 lines, the write frequency is set to 60 Hz, scanning signals of the first field are set to the numbers 23, 24 to 261 and 262, and scanning signals of the second field are set to the numbers 285, 286, 287 to 523, 524 and 525, for example, the signal having the scanning signal number 23 is written in two lines of the 1st line and the 2nd line that are adjacent above and below, and a signal having the next scanning signal number is written in two lines successively by the same method, thus forming the first field by repeating the foregoing sequentially, and, on the other hand, in the second field, the signals are written successively by the same technique in two lines with a different combination from that of two adjacent lines that have been written in the case of the first field, thus being driven by a double speed line sequential driving system capable of inputting a signal to respective lines at an apparently double speed.
However, it cannot be said that the NTSC signal is put to practical use sufficiently in the present state of things in the conventional single line sequential driving system or double speed line sequential driving system such as described previously.
When it is intended to directly introduce an interlaced driving system in which the scanning line numbers of the first field are set to 23, 24 to 261 and 262, the scanning line numbers of the second field are set to 286, 287 to 524 and 525, and a write frequency of 30 Hz is set in the signal write of the first field and the signal write of the second field in the 480-line liquid crystal display unit as shown in FIG. 9, the display of the liquid crystal has to be held for 1/30 second, but the following advantages and disadvantages are produced with the above.
First, as the advantages, the vertical resolution is improved due to numerous number of lines, and moreover, it is possible to aim at lower power consumption as compared with the double speed line sequential driving and so on and to aim at achievement of low cost. However, the lowering of contrast and the increase of flicker are anticipated by the influence that the time of holding the voltage of the liquid crystal gets longer.
Accordingly, in future development of the liquid crystal display, the development aiming to make the most of performance and design technique of the TFT in the present state while reproducing the picture information of the NTSC signal sufficiently.
Further, in a TFT color liquid crystal display unit in general, the scanning line and the signal line are arranged on the side of the TFT substrate, a common electrode is arranged on the side of the color filter substrate, the scanning signal is applied basically to the scanning line, and a corresponding display signal is sent in the signal line so as to perform an action as a matrix, and thus, the pixel realizes a high picture quality by a charge holding action.
When alternating current driving for inverting the polarity of the display signal in every field as described previously in such a TFT color liquid crystal display unit, the signal sent from the signal line is inverted and inputted to a pixel, but there have been several systems for inversion of a signal. Field inversion for inverting the voltage polarity of all the pixels into the same polarity in the unit of field has been known as the simplest inversion system, but it is practically impossible to make the applied voltage completely symmetrical between positive and negative polarities, thus inducing the flicker frequently.
Therefore, signal line inversion in which the inverted pixel unit is produced with respect to every signal line as shown in FIG. 10A, gate line inversion in which the inverted pixel unit is produced with respect to every scanning line as shown in FIG. 10B and so on are adopted, but a dot inversion system in which the pixel is inverted in every adjacent dot as shown in FIG. 11 is also adopted for the purpose of solving crosstalk and insufficient write better than those systems.
When the double speed line sequential driving system is replaced with the interlaced driving thereby to adopt the inversion driving shown in FIG. 12 by way of experiment in the 480-line liquid crystal display unit obtainable in the present state under such circumstances, it has been ascertained that 80&gt;CR (contrast) is produced in a liquid crystal display unit which had 100&gt;CR (contrast). It is conceived to be caused by such a reason that effective voltage practically applied to the liquid crystal has dropped because the write frequency gets longer (twice). With respect to this point, however, it is possible to solve this problem easily when a liquid crystal which is capable of obtaining the same transmittance characteristic at lower voltage (a liquid crystal corresponding to low V.sub.th (threshold voltage)) is adopted.
However, it has been found that, when a 480-line liquid crystal display unit is driven by interlaced driving, fluctuation in an oblique stripe form such as a beat noise which cannot be dissolved even by the inversion driving shown in FIG. 12 is visible. Then the cause of generation of such an oblique stripe was analyzed.
The analyzed liquid crystal panel is a TFT liquid crystal display panel, which has the display capacity of 480 lines and is driven interlacedly.
To be concrete, as shown in FIG. 13, a transparent substrate 8 provided with a plurality of TFT circuits 5 and pixel electrodes 6 and a deflecting plate 7 is arranged opposedly at a predetermined space against a transparent substrate 12 provided with a common electrode 9, a color filter 10 and a deflecting plate 11 and a liquid crystal is sealed in a gap 13 between both substrates 8 and 12, thereby to form a rough structure.
The TFT circuit 5 of a liquid crystal panel used in this analysis has a structure that, as shown in detail with an equivalent circuit shown in FIG. 14, a plurality of scanning lines 15 and signal lines 16 are formed in a matrix form, a TFT body 17, a liquid crystal 18 as capacity and a storage capacity 19 are connected in a region surrounded by respective lines, and the storage capacity 19 is connected to the scanning line 15.
Next, as to the color arrangement of respective dots in red, green and blue (described as RGB hereinafter) of a color filter of this type of TFT liquid crystal display panel, a color filter 20 in which respective colors of RGB are arranged in a mosaic pattern form declining obliquely from left to right as shown in FIG. 15A, a color filter 21 in which respective colors RGB are arranged in a mosaic pattern form declining obliquely from right to left as shown in FIG. 15B, a color filter 22 in a vertical stripe pattern form in which respective colors of RGB are arranged in one vertical line as shown in FIG. 15C, and a color filter 23 in which respective colors of RGB are arranged in a triangle pattern form as shown in FIG. 15D are known, but the color filter 21 in the arrangement state of the dots of RGB shown in FIG. 15B has been adopted in the color filter 10 used in this analysis.
The arrangement of driving voltage polarity in a display state of the micro color filter when the TFT liquid crystal panel having the above-mentioned structure is used and interlaced driving and inversion driving shown in FIG. 12 are performed is shown in the order of driving sequence as FIGS. 16A, 16B, 16C and 16D. In these figures, particular attention is paid to a pixel G of high visibility, and a state that positive write has been made in the black dot-painted portions of the dots G of the micro color filter and negative write has been made in the portions with round marks of the dots G of the micro color filter is shown.
As it is apparent from these figures, when interlaced driving or inversion driving in the sequence shown in FIG. 12 of the TFT liquid crystal panel is performed, asymmetry of the voltage applied to the pixel and transmittance variation of one dot from the driving frequency cannot be negated by the transmittance variation of pixels therearound, and conversely, it is visible as fluctuation in a stripe form obliquely declining from right to left in the color filter arrangement of the mosaic obliquely declining from right to left. Further, this type of fluctuation in a stripe form is similar also in other color filter arrangements mentioned previously.